Cure and Cura grant healing based directly on the Magic of the caster. The higher your effective Magic stat, the more HP are healed.
Curasa and Curaja grant healing based on a percentage of the difference between the max HP and current HP of the target. In XIII, you get a very small boost to those percentages as your Magic increases; I believe the same is true in XIII-2, but I am not certain.
So you want to use Curasa and Curaja when your target has a large gap between their max HP and their current HP. Exactly how large the gap should be is determined by your Magic stat.